DESCRIPTION This is the first submission of a proposal requesting renewal of a long-standing grant from a productive senior investigator. It continues a series of studies which have investigated various aspects of exocytosis. The focus of this submission is CIRL, a receptor that recognizes alpha-latrotoxin, an active component of black widow spider venom. Alpha-latrotoxin is known to form non-selective cation pores in lipid bilayers, but in cell membranes requires some cofactor(s). CIRL is apparently critical in the ability of alpha-Ltx to bind to cell membranes, but the functional relationship of CIRL to channel pore formation by alpha-Ltx is not known. A key question to be addressed is whether alpha-Ltx forms pores directly simply by tethering to CIRL, or whether the pores are formed through the activation of CIRL. In that regard, the applicant has shown that CIRL resembles a 7-TM type receptor. CIRL has a cleavable extracellular alpha-Ltx binding domain, and a 7-TM domain that has moderate identity and homology with secretin receptors. CIRL can also bind to syntaxin. This last feature forms the basis for studies designed to evaluate the effects of CIRL on both G proteins, and on syntaxin. This application represents a broad based structure-function study of CIRL.